1. Field of the Invention
The present invention generally relates to a dynamic pressure gas bearing structure, and more specifically, it relates to a dynamic pressure gas bearing structure for supporting a rotor which is rotated at a high speed.
2. Description of the Background Art
Following improvement in storage capacity and reduction in access time of a rotation driving part of a magnetic recording apparatus such as a hard disk driver (hereinafter referred to as "HDD"), for example, a high rotational speed and high rotational accuracy which are responsive thereto are required for a driving spindle motor of the HDD in recent years. In order to rotate such a precision motor requiring a high rotational speed and high rotational accuracy at a higher speed, employment of an air bearing (dynamic pressure gas bearing) for the rotation driving part has been proposed. In such a rotation driving part employing the air bearing, air is forcibly introduced into at least a gap between a radial gas bearing body and a rotor upon rotation of the rotor. Thus, air pressure is increased in the gap to rotate the rotor at a high speed. Thus, maintenance of the rotational accuracy is expected also during high-speed rotation, due to the employment of the air bearing.
In the aforementioned radial gas bearing, a wedge-shaped clearance is defined by eccentricity of a shaft body in the bearing body, as described in "Gas Bearing" by Shinichi Tohgo, Kyoritsu Shuppan (1984), for example. Air passing through the wedge-shaped clearance is compressed to generate pressure. Thus, the shaft body and the bearing body can be supported in a non-contact manner. According to "About Whirling of Gas Bearing" by Atsunobu Mori, "Junkatsu" Vol. 20, No. 7 (1975), pp. 481 to 488, however, a cylindrical journal bearing which is brought into an unloaded state in case of supporting a vertical shaft or the like presents an instable phenomenon called "whirling". In this phenomenon, the shaft is pressed against the bearing surface by centrifugal force to whirl in the interior of the bearing at any rotational speed. In the cylindrical journal bearing, the bearing center deviates from the rotation center due to a static load to generate pressure in a single portion, thereby bringing stable rotation. When the cylindrical journal bearing is applied to a vertical structure of supporting a vertical shaft or the like, the bearing is brought into an unloaded state and hence the pressure generating part is changed by disturbance, to instabilize the rotation.
When the aforementioned dynamic pressure gas bearing is applied to the HDD, the aforementioned factor of instability must be eliminated in consideration of positional accuracy of the rotor which is regarded as important.